I am on medicines for panic attacks since the last six yearsbut i still get very severe panic attacks suddenly, for noapparent reason. can i get panic attacks with no triggers?

Yes: Panic attacks can come out of the blue with no apparent trigger. But at some level there is a trigger activating the hpa axis. Basically this is fight-flight mode when danger is perceived, even if only at the level of deeper brain structures including the amygdala. Besides medication, there are various techniques that can help: deep breathing, mindfulness practice, energy tapping....

Yes: A classic symptom of panic attacks is to wake up from sleep with a panic attack. Often the attacks appear to have no trigger, but the underlying dysregulation of the stress response system is easily triggered into a panic attack. If the person also avoids things/activities for fear of having a panic attack, then panic disorder becomes the diagnosis. Seek cognitive behavioral therapy for panic.

Maybe time for chang: Lexapro never got approved for panic disorder because it failed trials in the us. You don't state what your Klonopin doses but perhaps it needs to be adjusted. I would recommend that you talk to your psychiatrist about a different antidepressant. Paxil (paroxetine) is very potent but is loaded with sexual side effects. Prozac started at a very low doses and then increased as tolerated can be very helpful. Cognitive behavioral therapy is necessary. Sometimes Klonopin simply wears off and needs to be carefully replaced with another benzodiazepine. Best wishes to you and i hope you and your neuropsychiatrist can find an adequate solution for you.